Federal and state governments have imposed increasingly strict regulations over the years governing the levels of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NO.sub.x) pollutants that a motor vehicle may emit to the atmosphere. One approach to reducing the emissions of these pollutants involves the use of a catalytic converter. Placed within the exhaust gas stream between the exhaust manifold of the engine and the muffler, the catalytic converter is one of the several emissions control devices typically found on a motor vehicle.
The catalytic converter is essentially a reaction chamber that contains an oxidation catalyst, typically in the form of one or more monolithic substrates, coated with a high surface area ceramic wash-coat and one or more precious metals such as Platinum, Palladium or Rhodium. When the engine is running, the exhaust gases from the exhaust manifold flow through the converter and pass heat to those composite materials housed within it. Once heated to a suitably high temperature, the composite materials convert a large percentage of the pollutants in the passing exhaust gases to carbon dioxide (CO.sub.2), water (H.sub.2 O) and other benign substances. Until the converter is brought up to operating temperature, however, its composite materials do not operate as effectively. As is well known, the catalytic converter is particularly inefficient when it is at its coolest, just after the engine is started cold. Consequently, absent other means of reducing such emissions while the engine and converter are warming up, a significant percentage of the pollutants would pass to atmosphere until the catalytic converter is sufficiently heated to operate satisfactorily.
One approach that has been proposed to reduce the emission of HC, CO and NO.sub.x pollutants while the exhaust system is cold is to use a second catalytic converter, often referred to as a warm-up converter. The warm-up converter would be small in size and located near the engine so that it could warm-up quickly. It would employ composite materials (i.e., a substrate, an oxidation catalyst and catalytic material coating) specially formulated to reach operating temperature quickly, thereby quickly rendering the warm-up converter capable of efficiently converting the pollutants in the exhaust gas. This is significant, as most of the pollutants are produced during the first minute or two after the engine is started. Until the engine and exhaust system have warmed to the point at which the conventional converter is operating more effectively, the exhaust gases during this "warm-up period" would be routed into the warm-up converter to remove the pollutants from the exhaust gases.
Given its proximity to the engine, the warm-up converter will generally not be able to withstand continuous exposure to certain harmful poisons carried by the exhaust gases. In particular, engine oil that may have been burned in the combustion chambers will be carried away by the exhaust gases into the exhaust system. Certain compounds in the oil, such as zinc-dithio-phosphate, will gradually coat the catalyst in the warm-up converter and soon render it ineffective. Prolonged exposure to the exhaust gases will therefore prematurely degrade the composite materials inside the warm-up converter.
A solution to this problem would be to strategically place an exhaust control valve within the exhaust system. Controlled by the engine control module (ECM) or other control component with feedback from a suitable sensor, the exhaust control valve can be automatically opened to allow exhaust gases to flow through the warm-up converter during the warm-up period and closed to prevent such flow afterward. By switching the flow of the exhaust gases away from the warm-up converter after the warm-up period, the exhaust control valve would then protect it from the relatively high temperatures and the harmful compounds carried by the exhaust gases. This tends to keep the warm-up converter free of poisons and highly effective during the warm-up period. After the warm-up period, the conventional converter due to its large size best treats the HC, CO and NO.sub.x pollutants. The large size of the conventional converter makes it more resistant to such poisoning.
This approach makes best use of both converters. The warm-up converter operates with peak efficiency quickly due to its close proximity to the engine during the critical warm-up period. Thereafter, only the conventional converter treats the exhaust gases. While flowing through the section of the exhaust pipe leading to the conventional converter, the exhaust gases are lowered in temperature somewhat. Soon operating within the desired temperature range, the composite materials in the conventional converter efficiently treat the pollutants in the exhaust gases. Deployed together in this control scheme using the exhaust control valve, the two catalytic converters reduce the HC, CO and NO.sub.x emissions far better than the conventional converter can alone. This is because they collectively treat the exhaust gases over more of the engine operating time than the conventional converter can by itself.
In such a control scheme, the exhaust control valve must be capable of operating over a wide range of temperatures, for example, from below 0.degree. C. to over 1000.degree. C. In particular, the valve must not stick or bind at any temperature within that range. It must open completely to let the exhaust gases flow through its flow passage. Conversely, it must close with a seal that is sufficient to prevent the exhaust gases from entering the warm-up converter after the warm-up period. Furthermore, the exhaust control valve must not allow exhaust gases to leak outside the exhaust system through its various joints.
A traditional butterfly valve would generally not be suitable for use as an exhaust control valve. The valve body for this type of valve defines a cylindrical inner bore that serves as a flow passage. Through the outer sidewall of the body is defined an opening through which a stem or shaft protrudes perpendicularly into the flow passage. Within the flow passage is placed a circular valve plate. Also referred to as the butterfly, the valve plate typically has an elongated slot formed within it into which the shaft is securely attached. Controlled in a known manner, the shaft can be rotated so that the valve plate, or butterfly, pivots within the flow passage between the opened and closed positions.
The butterfly valve is designed so that when the valve plate is pivoted to the closed position its perimeter contacts the cylindrical inner bore of the valve body. Experience has shown that this design is inadequate for the environment that an exhaust control valve must endure. In particular, the valve plate and body expand and contract, so much so that it is difficult to achieve a good seal over the temperature range to which the valve would be exposed. This design requires the valve body, valve plate and various other parts to be manufactured to meet extremely tight tolerances, thus increasing the cost. Extreme accuracy must be exercised in fitting the valve plate to the inner bore. If the dimensions of these critical parts are not within design tolerances or thermal expansion and contraction are not accounted for, the valve plate in the closed position will fail to adequately seal off the flow passage. In addition, the close proximity of the perimeter of the valve plate to the inner bore of the body tends to cause particulate matter to build up on those surfaces, further impeding the operation of the valve. Consequently, butterfly valves tend to leak excessively.
Various types of valves have been proposed for use as an exhaust control valve. One type of valve designed for this purpose is disclosed in U.S. Pat. No. 5,630,571 to Kipp et al., hereby incorporated by reference into this document. FIGS. 1, 2 and 3 herein show the exhaust control valve taught in that reference. Specifically, the Kipp et al. valve has a flow housing 10, a stem housing 26 and a mounting bracket 28. A hole 30 in the bracket 28 allows an actuator to be to attached the exhaust control valve. The valve mounts to the exhaust system through inlet and outlet pipes 112 and 114. The Kipp et al. valve is described in the following paragraphs, as it is an example of the many types of exhaust control valve that can be improved by the invention disclosed within this document.
FIGS. 1 and 2 best illustrate the structure of the stem housing 26. Through its components, the stem housing 26 provides a mounting for the shaft that is to be pivoted within it. This mounting includes seals that prevent leakage of exhaust gases from the flow passage of housing 10 through the stem housing 26 to the environment outside the exhaust system.
The stem housing 26 defines a first bored opening 79 into which has been press fit a first bushing 78. A second bored opening 75, axially adjacent to and coaxial with the first bored opening 79, provides an interior working space 74 between the first bushing 78 and a second bushing 66. Inside space 74, a steel washer 72 is affixed to a shaft 60 so that it is located between the two bushings 78 and 66 when the valve is assembled. Washer 72 has a flat end 71 facing bushing 78 and a convex-shaped end 80 facing bushing 66. A ceramic washer 70 is located between washer 72 and bushing 66. It has a concave surface 82 shaped to receive the convex end 80 of washer 72. Washer 70 is neither rigidly attached to stem housing 26 nor to shaft 60.
The concave surface 82 of washer 70 and the convex end 80 of washer 72 engage to form an annular socket joint that seals when force in the axial direction of shaft 60 presses the steel washer 72 against the ceramic washer 70. A radially extending portion 68 of bushing 66 is press fit into a third opening 69 bored into stem housing 26. Bushing 66 with its extending portion 68 thus not only prevents washer 70 from moving out of stem housing 26 but also seals off the interior working space 74. Ceramic washer 70 provides a tight seal against both steel washer 72 and the flat annular shaped extending portion 68 of bushing 66. It also prevents fusion of the various sealing parts when the valve is operating at high temperatures.
Bushing 66 also helps provide a stable mounting for the shaft 60. A flat retainer 62 is press fit via hole 102 onto shaft 60 and together with bearing 66 sandwiches a wave washer 64. Retainer 62 is positioned so that wave washer 64 is compressed and thereby exerts along its axis a spring force between bushing 66 and retainer 62. This provides a biasing force on shaft 60 in the direction of arrow 61. The spring force provided by wave washer 64 biases washer 72 against washer 70, thereby sandwiching washer 70 between washer 72 and the extending portion/end cap 68 of bushing 66. The gap between the interior surfaces of bushings 78 and 66 and the shaft 60 must be wide enough to allow for thermal expansion of shaft 60 and for adequate clearance under all manufacturing conditions. This prevents shaft 60 from being in airtight engagement with bushing 78 or bushing 66. As biased by wave washer 64, however, the washers 70/72 and end cap 68 together provide the seal that prevents exhaust gas from escaping to the outside atmosphere.
FIGS. 1 and 2 show that the retainer 62 is preferably part of an actuator arm 100. Actuator arm 100 thus not only keeps wave washer 64 in place but also acts as a lever to which an actuator of some type connects. Attached to the lever, pin 104 serves as an easily accessible site to which a pneumatic or solenoid based actuator can link. Ultimately controlled by the ECM or like component, the actuator is what actually moves the actuator arm 100 via pin 104. The actuator arm, in turn, is what pivots the shaft 60 and the valve plate 90 within the flow passage of housing 10 between the opened and closed positions.
The flow housing 10 defines a cylindrical inner bore that serves as the flow passage. The interior wall of the cylindrical bore can be viewed as comprising a wall portion 16 and a wall portion 18 separated by an imaginary vertical plane that passes through a pivot axis 15 of shaft 60, as best viewed from the perspective of FIG. 1. The thickness of housing 10 is denoted by numeral 32 in FIG. 3. Extending radially inward from the interior wall by an additional thickness 22 are two seat arcs 34 and 36. Each seat arc is essentially a thickened portion of the interior wall. The two seat arcs 34 and 36 are equidistant from the vertical plane, spaced apart from each other by a distance 35 that is approximately equal to the thickness of plate 90. Seat arc 34 extends inwardly from wall portion 18 over almost 180.degree. of arc along the interior wall below a horizontal plane that passes through the pivot axis 15. Similarly, seat arc 36 extends inwardly from wall portion 16 over almost 180.degree. of arc along the interior wall above that horizontal plane. Manifested as a circular ledge, the inner side of seat arc 34 serves as a valve seat 12. Likewise, the circular ledge or inner side of seat arc 36 serves as a valve seat 14.
The flow housing 10 may be installed within an exhaust system with or without the use of connecting tubes 112 and 114. Connecting tube 112 may be seated in one end 40 of housing 10 against a tube seat formed by outer side 48 of seat arc 36 and its narrower arcuate extension, outer side 38. As best shown in FIG. 1, outer side 38 extends nearly 180.degree. of the arc within end 40, and outer side 48 extends the other approximately 180.degree. of arc within end 40. Similarly, connecting tube 114 may be seated in the other end 42 of housing 10 against a tube seat formed by both an outer side 52 of seat arc 34 and its narrower extension, outer side 44, as best shown in FIG. 3.
Through the cylindrical sidewall of housing 10 is defined an opening, designated 20/24 in FIG. 3. This opening extends through the seat arcs 34 and 36, as is also shown in FIG. 2. Specifically, numeral 20 also denotes a semicircular gap in seat arc 34 that has been bored out of, or otherwise omitted from, housing 10. Similarly, numeral 24 denotes a semicircular gap in seat arc 36 that has been omitted from housing 10. Together, these omitted portions comprise the opening 20/24 in the sidewall of housing 10 through which shaft 60 protrudes from stem housing 26 perpendicularly into the flow passage.
Within the flow passage is situated the circular valve plate 90. The valve plate 90 has an outer perimeter 92. As best shown in FIG. 2, the valve plate has an elongated slot that fits around the end of shaft 60. Shaft 60 may mount to one side 91 of plate 90 or even extend to its other side 93. Although other known methods can be employed, the valve plate is typically affixed to shaft 60 by laser welding, a quick method that prevents warping of plate 90 as it avoids generation of excess heat. Controlled using an actuator as described above, the shaft 60 can be rotated so that the valve plate 90 pivots within the flow passage between the opened and closed positions.
Just inside its perimeter, valve plate 90 has a pair of semi-annular facets, or arcuate sectors, 94 and 96, each extending approximately 180.degree. of arc on the outermost circular edges of the disk. The arcuate sectors, however, are located not only on opposite faces of valve plate 90 but also on opposite sides of pivot axis 15. Specifically, arcuate sectors 94 and 96 seat on valve seats 12 and 14, respectively, when the valve plate 90 is in the closed position within the flow passage. As best shown in FIGS. 2 and 3, the arcuate sectors 94 and 96 cover most of the surface of their respective seats 12 and 14 when the valve plate is closed. Because the diameter of plate 90 is selected to be smaller than the diameter of the inner bore of housing 10, the thermal expansion of the plate 90 and housing 10 need not exactly match. This design thus allows for some expansion and contraction of the valve plate 90 and housing 10 over the range of expected operating temperatures. The valve plate 90 will not stick or bind within the inner bore at any temperature within that range.
Assuming the internal surfaces and parts of the Kipp et al. valve are manufactured to meet design tolerances, the arcuate sectors 94 and 96 of valve plate 90 should seal fairly well against the valve seats 12 and 14. Because seats 12 and 14 extend radially inward a substantially equal distance over their respective 180.degree. arcs, no leaks of exhaust gas should occur between the perimeter 92 of plate 90 and the interior wall of housing 10. Nevertheless, the design is not completely impervious to leakage. Some leakage is to be expected through region 105. Referring again to the imaginary horizontal plane that passes through pivot axis 15, seat arcs 34 and 36 each fall short of that plane by a distance equal to roughly half of the thickness of plate 90. This region 105, shown in FIG. 1, allows allow room for valve plate 90 to be pivoted to the fully opened position. When the valve plate 90 is closed, however, exhaust gas can leak through this region. The magnitude of the leak, however, would be relatively inconsequential. The slight leakage would pose no substantial risk of damage to the warm-up converter with which the exhaust control valve of U.S. Pat. No. 5,630,571 is used.
Despite its advantages over traditional butterfly valves, the Kipp et al. exhaust control valve still has its drawbacks. To achieve the required low level of leakage needed to protect the warm-up converter after the warm-up period, certain parts of the valve must meet extremely tight tolerances. In particular, the position at which the seat arcs 34/36 are formed within the inner bore of housing 10 must be controlled relative to the shaft 60 and the valve plate 90 to which it is attached. The distance between seat arcs 34 and 36, the flatness of both the seats 12/14 and the arcuate sectors 94/96, and the thickness of plate 90 must also be kept within extremely close limits.
Moreover, the flow housing 10 is made from a single piece of metal, typically stainless steel. This makes it very difficult to machine the valve seats 12 and 14 on seat arcs 34 and 36, respectively. Specifically, the rotating milling tool used to machine the seats 12 and 14 must be of a very small diameter. This is because the ends of the two seat arcs 34 and 36 are so close together, as evidenced by region 105 in FIG. 1. The milling tool must have a small diameter so that it can machine the surface of seat 12 at the end of seat arc 34, and yet not bump into and/or damage the other seat 14 at the end of seat arc 36 that lies in such close proximity, and vice versa. In this confined region, the end of each seat arc tends to get in the way of the milling tools used to machine the seats 12/14.